Devices for measuring fluid density are known. Such devices typically use a density transducer comprising a transducer head containing a vibrating cylinder. The cylinder is caused to vibrate by a magnetic field generated by an alternating current in a coil surrounding the cylinder. The density transducer cylinder, immersed in the fluid to be measured, will have a resonant frequency of vibration that is dependent upon the density of the surrounding fluid. Measurement of this resonant frequency by electronic circuitry permits calculation of the fluid density. An example of a density transducer is the Avionic Liquid Density Transducer, 7824 Series, manufactured by Solartron, a division of Schlumberger Electronics (UK) Ltd.
The measuring electronics for this device typically have been required to be physically located near (e.g. within 2 to 3 meters of) the density transducer itself. A common arrangement has been to mount the electronics in a module that is located directly opposite the transducer head, on the outside of the fuel tank. The reason for such close mounting has been that, because of impedance, the resonance of the circuit is affected by the length of the electrical leads running to the transducer, and the electronics typically cannot adequately compensate for such effect on resonance. Without proper compensation, an inaccurate density reading will be produced.
The disadvantages of such an approach are apparent, particularly in harsh environments such as those encountered by aircraft fuel tanks which are often subject to vibration and temperature and pressure extremes. Locating the measuring electronics on or near the fuel tank not only may be physically awkward from a design, engineering or space standpoint, but also increases the risk of malfunction in the electronics due to the harsh environment, and may render the electronics difficult to repair because of inaccessability.